1. Field of the Invention
The present invention relates to a polymer electrolyte fuel cell for generating electricity using an electrochemical reaction for use in an electric automobile, for example, and to systems therefor.
2. Description of the Related Art
As is well known, fuel cells are typical electrochemical devices in which chemical energy is converted directly to electric energy by placing a pair of electrodes in contact by means of an electrolyte, supplying fuel to one of the electrodes and an oxidant to the other electrode, and allowing the electrochemical oxidation of the fuel to proceed within the cell.
There are several types of fuel cell depending on the electrolyte used, but in recent years polymer electrolyte fuel cells using polymer electrolyte membrane as an electrolyte have attracted attention as fuel cells providing high output.
In a fuel cell, when hydrogen gas is supplied to the fuel electrode and oxygen gas is supplied to the oxidant electrode and electric current is removed by an external circuit, the following reactions occur: EQU Fuel electrode reaction: H.sub.2.fwdarw.2H.sup.+ +2e.sup.- (1) EQU Oxidant electrode reaction: 2H.sup.+ +2e.sup.- +1/2O.sub.2.fwdarw.H.sub.2 O (2)
If catalysts such as platinum, etc., on the electrodes work effectively, the reactions proceeds smoothly with hardly any overvoltage occurring in the above reaction (1).
However, when hydrocarbons such as methanol, etc., which are easy to handle are used as the fuel, the fuel is first reformed to hydrogen in a reformer by a reaction such as reaction (3) below. EQU Reforming reaction: CH.sub.3 OH+H.sub.2 O.fwdarw.3H.sub.2 +CO.sub.2 (3)
However, trace quantities of carbon monoxide may contaminate the fuel due to the shift reaction (4) given below. EQU Shift reaction: CO.sub.2 +H.sub.2.fwdarw.CO+H.sub.2 O (4)
One problem, particularly in electrochemical devices such as polymer electrolyte fuel cells which have a low operating temperature, is that the catalyst can be poisoned by the presence of a few tens of parts per million of carbon monoxide, lowering performance by increasing overvoltage from the reaction at the fuel electrode (anode).
Thus, in order to reduce the effects of carbon monoxide poisoning, many adaptations have been conventionally adopted. They basically fall into: the development of catalytic compositions less affected by carbon monoxide; and the development of methods for selectively removing carbon monoxide from the fuel.
Retention of high performance at carbon monoxide concentrations of up to 100 ppm has been reported for catalytic compositions using alloys of platinum (Pt) and ruthenium (Ru). ("Behavior of CO Poisoning on Electrocatalysts for Polymer Electrolyte Fuel Cell", Keynote Lectures of the Thirty-Fifth Battery Symposium in Japan, 3D19, pp 299 to 300 (1994))
Furthermore, reports of methods for selectively removing carbon monoxide from fuel include a method for removal by the addition of air into the catalyst layer (Canadian Patent No. 1,305,212), a method for introducing trace quantities of air into the fuel of a fuel cell ("Performance of Polymer Electrolyte Fuel Cells with Three Dimensional Bonding Method", Keynote Lectures of the Thirty-Sixth Battery Symposium in Japan, 1C07, pp 225 to 226 (1995)), etc.
In selective oxidation by the introduction of air in this manner, it has been confirmed that by oxidizing the carbon monoxide in the fuel carbon monoxide concentrations can be reduced to levels which do not cause poisoning.
However, when the carbon monoxide concentration is high, even the performance of Pt-Ru alloys is unstable, and so improvements in catalytic composition are still insufficient as a countermeasure to poisoning.
Furthermore, shortcomings remain in selective oxidation by the introduction of air, such as large amounts of hydrogen being simultaneously consumed by combustion, or performance being reduced by dilution of the fuel gas due to nitrogen being the major component of air, or corrosion occurring in the cell elements due to hydrogen gas being rarefied by the residence of inert gas.
Furthermore, if the amount of air is too small, carbon monoxide is not removed sufficiently, making it important to control the amount of air in response to the amount of fuel and the concentration of carbon monoxide, but control has been difficult because there has been no easy way to ascertain the concentration of carbon monoxide.
Now, Japanese Patent Laid-Open No HEI 7-105967 describes the construction of a fuel cell in which a carbon monoxide-removing fuel cell is disposed in a fuel supply passage of a main fuel cell, and a fuel discharge passage from the carbon monoxide-removing fuel cell is connected to the fuel supply passage of the main fuel cell.
Reformed gas composed mainly of hydrogen is first supplied to the carbon monoxide-removing fuel cell and hydrogen fuel is consumed by the above reaction (1), but carbon monoxide contained in this fuel simultaneously adsorbs onto the catalyst on the fuel electrode. At the same time, the above reaction (2) is performed at the oxidant electrode by supplying oxygen, the amount of oxygen passing through the electrolyte membrane and reaching the fuel electrode on the other side being controlled by adjusting the oxygen supply pressure. The following oxidation reaction (5) occurs on the fuel electrode, and since the poisoning carbon monoxide adsorbed onto the fuel electrode is removed by desorption as carbon dioxide, only the trace quantities of carbon monoxide remaining after adsorption and removal in the carbon monoxide-removing fuel cell are introduced into the main fuel cell. EQU Oxidation reaction: CO+1/2O.sub.2.fwdarw.CO.sub.2 (5)
However, this still leaves the problem of loss of current generating efficiency due to a portion of the hydrogen in the fuel being consumed by reaction with oxygen at the same time as the oxidation reaction (5) occurs. Another problem is that extra power is required to provide the pressure to make the oxygen pass through.